Human prolactin secretion rises dramatically at night. For more than 20 years, this rise has been considered to be sleep-dependent, because it usually coincides with sleep, it can be delayed by delaying sleep and it can be clocked by preventing sleep. Our recent work suggests, however, that the nighttime rise of PRL secretion is triggered by quiet rest, rather than sleep. The purpose of the present study is to attempt to confirm this preliminary finding in a controlled experiment in which effects on PRL secretion of undisturbed rest, and of rest disturbed conversation, will be compared in individuals lying in bed in the dark. The results of the study may prove to be relevant to the neurobiology of psychiatric syndromes, many of which are characterized by an absence or an excess of quiescence and the tranquility. Since animal research has already shown that PRL can induce quiescent states, it is also possible that pharmacological manipulations of PRL or PRL receptors might prove useful as treatments for these syndromes. More generally, confirmation that undisturbed rest stimulates PRL secretion would open up a new area of human behavioral neuroendocrinology whose relevance to human health and well-being could be a focus for further investigation. Subjects are admitted to an inpatient unit of the NIH Clinical Center for four consecutive nights. Each night, from 6:00 pm to 8:00 am, they are asked to lie in bed in a dark room and to rest and sleep (our imposition of this schedule is simply a device that allows our subjects to remain awake in the dark without trying to do so and enables us to dissect away effects of sleep from the effects of undisturbed rest on PRL secretion). On Night 3 or 4, as determined by balanced-order random assignment, they are asked to engage in conversation with a staff member during the first half hour of the dark period (6:00 pm to 6:30 pm) while lying in bed in the dark room. On Night 1 (an adaptation night) and on both Nights 3 and 4, an intravenous catheter is inserted in a forearm vein and blood samples are drawn at 5:30 pm -(30 min), 6:00 pm (0 min) and 6:30 pm (+30 min). Later, levels of PRL are measured in plasma obtained from the samples obtained on Nights 3 and 4. On the basis of our preliminary findings, we expect that +30 PRL levels, relative to -30 min and 0 min baseline levels, will increase in the Rest (no conversation) Condition but not in the Disturbed Rest (conversation) Condition.